1. Field of the Invention
The invention relates to a diode-pumped solid-state laser (DSSL) with a pump light source. The light source has a diode stack, with a laser rod and a pump cavity. The center of the emitting face of the diode stack has a defined position in space relative to the laser rod.
2. Description of the Related Art
Such a DSSL is disclosed by the publication "Compact 170 W continuous-wave diode-pumped Nd:YAG rod laser with a cusp-shaped reflector" by T. Brand (Optic Letters Vol. 20, p. 1776, 01.09.1995).
Because of their significant advantages, DSSLs are taking over to an increasing extent from lamp-pumped lasers in many fields of application. Through the technological possibility of arranging a multiplicity of laser diodes in a row (linear diode array) or a plurality of such linear diode arrays above one another as a stack (diode stack), it is possible to form pump light sources with an extremely wide range of pump power. Further, DSSLs have high electro-optical efficiency, long life, better beam quality because of the smaller heat input into the active element, and such systems can be designed more compactly because of their low thermal dissipation.
There are, however, a few disadvantages.
The most significant disadvantage results from the differing divergence of the diode radiation in the direction of the fast axis (perpendicular to the pn junction) and in the direction of the slow axis (parallel to the pn junction) of the laser diodes. The radiation from the emitting face of a diode stack with a length equal to the linear diode array length and a width equal to the stack height is correspondingly directional radiation with a divergence angle of about 15.degree. across the width (slow axis) and about 50.degree. across the length (fast axis) of the emitting face (pump radiation).
In order to obtain a high input coupling efficiency for the pump radiation, the latter must be adjusted in terms of its direction and position with respect to the active element to be pumped, or where applicable with respect to input coupling optics which may also be a pump cavity. This means, in practical terms, that the pump light source must be arranged in a particular relative position with respect to the active element or with respect to the coupling optics. This requirement is the main reason why the pump light source in a DSSL is not easily interchangeable, as is usual in the case of lamp-pumped solid-state lasers and is regarded as a particular advantage.
A further disadvantage is the electrostatic sensitivity of the diodes, which are straight away irreparably damaged by an electrostatic discharge. There are also stringent requirements on the accuracy of the temperature of the cooling water and the cleanliness of the environment. Condensation of water on the diode likewise leads to their more rapid degradation.
The aforementioned basic requirements, in particular the need to adjust the defined relative position, make simple interchanging of the pump light source impossible in the case of most known DSSL arrangements.
Transverse pumping of the active element without coupling optics, because of the pronounced divergence of the pump radiation in the direction of the fast axis, is only possible with individual linear diode arrays which are arranged radially distributed close around the active element. The number of linear diode arrays can be increased if a cylindrical lens is arranged as coupling optics in front of each of them, for example, in the linear array direction.
In such solutions, the pump light source is correspondingly a sum of many separately arranged individual linear diode arrays, each linear diode array needing to be adjusted on its own with respect to its coupling optics. Design solutions in which the user can himself carry out interchanging are not possible.
For longitudinal and transverse pumping, it is known to combine the radiation from individual or even a plurality of linear diode arrays using sometimes very complicated optics, and to shape it, in such a way that it can be coupled into an optical fiber.
DSSL arrangements with fiber-coupled linear diode arrays are for example known from U.S. Pat. Nos. 5,127,068, 5,436,990 and 5,446,749 . These systems are suitable for having design configurations such that the linear diode array with the coupling optics and the optical fiber are arranged in a compact pump module which, should need be, can be completely interchanged. Since, essentially, only the input coupling of the radiation from individual, or only a few, linear diode arrays into an optical fiber is possible, the possibility of scaling the pump power is very limited. Further, such pump modules are very expensive since they also contain complete coupling optics.
The article "Compact 170 W continuous-wave diode-pumped Nd;YAG rod laser with a cusp-shaped reflector" by T. Brand (Optic Letters Vol. 20, p. 1776, 01.09.1995) describes a transversely pumped DSSL in which the pump radiation is essentially reflected across a pump cavity into a laser rod. Using this pump cavity, as described by way of example in the article, it is possible to couple the pump radiation of a diode stack with a size of 1 cm (linear diode array length).times.4.5 cm (stack height) into a laser rod with 4 mm diameter. This extreme stack height permits hitherto unknown scaling of the pump power using a single pump light source. In order to pump the laser rod effectively, the center of the emitting face of the diode stack should be arranged opposite the center of the laser rod and at a distance from the pump cavity such that it is maximally illuminated but without having a beam-limiting effect on the pump radiation.
The optical principle of coupling the pump-light radiation in through a reflecting pump cavity does not place such extremely stringent requirements on the accuracy of the adjustment of the pump light source as are made in the case of the known input coupling by means of transmissive input coupling optics. Further, such an arrangement is particularly advantageous because basically no changes need to be made to the design dimensions of the pump cavity, which may be configured as diffuse or specularly reflective and in different geometries, as a function of the desired pump power, which can be varied by means of the stack height.